In version 7 some messages will not change to ‘read’ even after they have been put in the trash folder. They are always from particular senders. I have the delay for showing ‘read’ set at 0.
This delay of 0 does not work if the message still has content to download from the Internet or the mail server. The delay of 0 will only begin once the message is completely downloaded.
What happens if you individually mark as read, or mark the folder as read?
To the best of my knowledge these messages are complete and do not still have content remaining to be downloaded. I have manually deleted them so I cannot check now. I have not tried to manually mark them as read when they are in the inbox. It is just annoying as usually messages automatically change to read when they are highlighted. I agree that that might be a way around it.